Impact of egg predation by Noctiluca miliaris on the summer development of copepod populations in the southern North Sea

Abstract

Development of zooplankton in summer and mechanisms that control population dynamics of copepods were studied in the Dutch coastal area of the North Sea with particular attention to the significance of predation as a regulating factor. Among predators the zooflagellate Noctiluca mihans, whlch IS known to feed on copepod eggs, played a major role. During Noctduca blooms in July often high percentages (up to 75 %) of the total number of copepod eggs in the water column were found in this predator's cells. However, the eggs appeared to be digested very slowly and, as a consequence, quantitative estimates of predation pressure by N. miliaris (based on abundance, number of ingested copepod eggs per cell and digestion time) came out lower than expected. The importance of this predation was judged by comparing it to daily copepod egg production. The maximum percentage of eggs cleared by N. miliaris before hatching was estimated at 50 %. Although the impact of N. rnilians predation on the recruitment of nauplii may thus be considerable, large fluctuations in natural egg densities are not likely to be caused by this predation, but may be attributable to changes in individual copepod egg production. Our data suggest that, during deteriorating food conditions in summer, egg production can fall by a factor of at least 7 to 10 and that therefore food should be considered a key factor regulating nauplii recruitment.